Case reports have suggested that obstructive sleep apnea (OSA) may be a frequent finding in patients with cluster headache (CH) and that treatment of sleep apnea in such patients may improve the headache syndrome. We will perform cross-sectional studies and a clinical trial to 1) assess the frequency of OSA in a sample of patients with active CH, 2) examine physiological mechanisms that may link the two disorders, and 3) test whether treatment for OSA improves CH and quality of life in patients who have both conditions.